Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle span and low self-discharge are commercially available and widely used.
Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
Among these cathode active materials, LiCoO2 is the most generally used owing to superior physical properties such as superior cycle characteristics, but has low stability and is costly due to resource limitations of cobalt used as a raw material, thus disadvantageously having limited mass-utilization as power sources for electric vehicles.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 advantageously are redundant resources and use environmentally friendly manganese, thus attracting much attention as a cathode active material alternative to LiCoO2. However, these lithium manganese oxides have disadvantages of low capacity and bad cycle characteristics.
On the other hand, lithium nickel oxides such as LiNiO2 are cheaper than the cobalt oxides and exhibit higher discharge capacity, when charged to 4.3V and reversible capacity of doped LiNiO2 reaches about 200 mAh/g which is higher than LiCoO2 capacity (about 165 mAh/g). Accordingly, in spite of slightly low average discharge voltage and volumetric density, commercial batteries comprising LiNiO2 as a cathode active material have improved energy density and a great deal of research on these nickel-based cathode active materials is thus actively conducted in order to develop high-capacity batteries. However, the problems of LiNiO2-based cathode active materials, including high preparation costs, swelling caused by gas generation in batteries, low chemical stability and high pH have not been solved yet.
In this regard, many conventional techniques focus on properties of LiNiO2-based cathode active materials and methods for preparing LiNiO2 and suggest lithium transition metal oxides wherein nickel is partially substituted by other transition metals such as Co or Mn.
Meanwhile, lithium transition metal oxides used as cathode active materials are in the form of a single particle (referred to as a “primary particle”) forming an independent structure, or of an aggregate particle (referred to as a “secondary particle”) wherein two or more primary particles form an independent structure.
For such particle shapes, when an average particle diameter of primary particles (referred to as “average primary particle diameter”) is large, the press density of electrodes can be improved and capacity of secondary batteries can thus be considerably improved. In addition, variations in specific surface area with passage of time are decreased, making it easier to handle particles (in the form of a powder) and improving processibility. Accordingly, a great deal of study to prepare lithium transition metal oxides which can exert desired performance through prevention of deterioration in tap density or optimization of particle shapes such as preparation of spherical particles based on control of factors such as particle size is underway.
However, in spite of these many advantages, nickel-based lithium transition metal oxides having an average primary particle diameter cannot be applied to general batteries. This is the reason that conventional techniques do not enable preparation of nickel-based lithium transition metal oxides having a large average primary particle diameter and a completely-grown crystal structure, thus exhibiting desired electrochemical performance.
In this regard, some related patents disclose primary particle diameters of Ni-, Mn- and Co-based lithium transition metal oxides.
For example, Japanese Patent Publication No. 2003-68299 discloses Li—Mn—Ni—Co oxides having an average primary particle diameter of 3 μm or less. When the average diameter is higher than 3 μm, an electrolyte solution cannot be permeated into the particles. Preferably, the average diameter is 1 μm or less. In addition, Comparative Example demonstrates that, when the primary particle diameter is 2 μm or higher, battery performance, such as discharge capacity and cycle properties, is deteriorated.
Korean Patent Laid-open No. 2008-0031424 discloses a cathode active material having a primary particle diameter of 0.1 μm to 3 μm. In addition, when the primary particle diameter is 3 μm or higher, a ratio of lithium ions which do not contribute to charge-discharge increases. Accordingly, it is preferred that the average diameter be 0.2 μm or less.
Also, many patents such as Japanese Patent Publication No. 2003-221236, Korean Patent Laid-open No. 2007-0097115 and Japanese Patent Publication No. 2008-84826 insist that an average primary particle diameter should be small.
However, as mentioned below, as a result of a variety of extensive and intensive studies and experiments, and analysis and consideration associated therewith, the inventors of the present invention confirmed that, in the case where the average primary particle diameter is several micrometers (μm) or higher and a stable crystal structure is thus realized, nickel-based active materials exhibiting superior electrochemical performance can be prepared.
Meanwhile, unlike the afore-mentioned patents, some related patents suggest an active material having a relatively large primary particle diameter.
Specifically, Korean Patent Publication No. 2004-0106207 discloses a cathode material wherein a plurality of primary particles are aggregated to form a secondary particle, and a length at which adjacent primary particles are bound to each other on the cross-section of secondary particle is 10 to 70% with respect to the total circumference of the cross-section of primary particles. In this patent, the primary particle diameter is within the range of 0.2 to 10 μm. However, this patent teaches that Comparative Example is performed at a temperature which is unsuitable to realize relatively normal electrochemical performance and the results thus obtained are compared with those of Example. When the sintering temperature is excessively low, the desired crystal structure is not formed and electrochemical performance of a material is deteriorated.
Accordingly, this patent does not teach a cathode material having a substantially large primary particle and exhibiting electrochemical performance.
Also, Japanese Patent Publication No. 2005-25975 suggests lithium nickel manganese cobalt-based oxides which are represented by Li1+xNi1−y−z−p MnyCozMpO2 (0≦x≦0.2, 0.1≦y≦0.5, 0.1≦z≦0.5, 0≦p≦0.2, 0.2≦y+z+p≦0.8) and have an average primary particle diameter of 3 to 20 μm. However, it is the most important that oxide particles have a large average primary particle diameter and superior crystallinity and thus exhibit superior electrochemical performance. The patent does not disclose such a crystal structure and electrochemical performance. In addition, the method for preparing the crystal structure comprises a multi-step heating process such as heating under non-oxidative atmosphere. The present invention enables preparation of a cathode material having a large and high average primary particle diameter and thus exhibiting superior electrochemical performance through conventional single heating.
Also, Japanese Patent Publication No. 2006-54159 discloses a cathode active material for non-aqueous secondary batteries which contains nickel and lithium as main ingredients, is represented by LixNi1−p−q−rCopAlqArO2−y (0.8≦x≦1.3, 0<p≦0.2, 0<q≦0.1, 0≦r≦0.1, −0.3<y<0.1) and is composed of single crystals having an average particle diameter of 2 to 8 μm. However, this patent discloses an active material which necessarily comprises inorganic chlorides or inorganic chloride oxides and teaches that, when an active material does not contain inorganic chlorides or inorganic chloride oxides or does not contain a sufficient amount thereof, growth of primary particles is inhibited.
As mentioned above, regarding the primary particle size, the related patents have different views. This is the reason that conventional patents focus on only the size of particles without considering the crystal structure of materials.
Accordingly, there is an increasing need for electrode active materials for lithium secondary batteries having a large average primary particle diameter and a stable crystal structure, thus exhibiting good electrochemical performance.